Anodic prevention of hydrogen embrittlement of metals



United States Patent 3,147,204 ANODIC PREVENTION OF HYDROGENEMBRITTLEMENT 0F METALS Spencer W. Shepard, Plainfield, and Charles K.Aldr ch, Buttzville, N.J., assignors to Chemical ConstructionCorporation, New York, N.Y., a corporation of Delaware No Drawing. FiledFeb. 25, 1960, Ser. No. 10,861 2 Claims. (Cl. 204-147) This inventionrelates to the prevention of metal failures due to absorption ofhydrogen and consequent embrittlement of the metal.

It has been found that hydrogen absorption by metals in contact withfluids which commonly cause hydrogen embrittlement is prevented bymaintaining an electrical potential between this metal and another metalimmersed in the fluid and functioning as a cathode. In this manner, themetal to be protected is maintained sufliciently anodic so that localcathodes can no longer form on its surface, with the fluid acting as anelectrolyte in the electrical system. The current is usually animpressed electromotive force, but could also be a galvanic current ifthe electrochemical potential of the two metals is sufliciently farapart. In general, this invention protects metals from the hydrogenembrittlement that commonly occurs where fluids are being handled orstored which tend to form films that allow or encourage the depositionof atomic hydrogen which forms on cathodic areas during the normalcorrosion process. Hydrogen embrittlement results when atomic hydrogenpasses freely through the lattice of the metal, until it combines toform molecular hydrogen or compounds in voids or at grain boundarieswithin the metal which develop pressure in the metallic voids. Thesepressures weaken the metal so that it becomes brittle and eventuallycracks.

The problem of hydrogen embrittlement is widely prevalent in thechemical and petroleum industry. Where this type of metal deteriorationtakes place, sudden cataclysmic cracking of vessels may occur withoutany prior warning. Thus hydrogen embrittlement may be distinguished fromthe more common corrosion situation i which there is a gradual failuredue to metal wastage or other surface attack. Hydrogen embrittlementresults from the actual absorption of hydrogen, usually atomic hydrogen,which is formed on metallic surfaces that become cathodic due toelectrochemical processes of corrosion. Usually the corrosion itself isnot a serious problem, however, the film of hydrogen deposited on themetal surface forms gaseous compounds with non-metallics in the grainboundaries of the metal. Thus the hydrogen penetrates through the metalsurface and is absorbed into the metal crystalline structure, resultingin deterioration of the strength of the metal due to embrittlement, andeventual failure of the chemical process vessel or other apparatusformed of the metal. Usually this failure is a sudden cracking orcollapse, with attendant disastrous effects.

The major metal requiring protection against hydrogen embrittlement issteel. Steel process vessels and storage tanks are widely used in thechemical and petroleum industries, and hydrogen embrittlement of suchvessels has been prevented in some cases by using a protective coatingwhich separates the steel from the electrolyte.

,This procedure is expensive and is only as reliable as the coating. Asan alternative, hydrogen embrittlement can sometimes be prevented byusing other metals. In some cases aluminum or stainless steel may besubstituted for steel, but this may be prohibitively expensive. Othermetallics which are sometimes susceptible to hydrogen embrittlementinclude the hardenable grades of stainless steel, copper and its alloys,and aluminum.

Although hydrogen embrittlement is widely encoun- 3,147,204 PatentedSept. 1, 1964 tered, in general this phenomenon occurs most frequentlyin systems involving weakly acid or alkaline solutions contaminated withsulfur compounds, cyanides, or compounds of arsenic or mercury. Thesecontaminants are fairly widespread, especially in the petroleum andchemical industries, thus numerous industrial systems may be protectedby the method of the present invention. Without so limiting theinvention, it appears from a theoretical standpoint that hydrogenembrittlement occurs Where the aforementioned contaminants are presentbecause the natural surface film which forms on the steel surface ismodified or eliminated due to chemical action of the contaminants, thuspermitting hydrogen penetration into the metal. Of course, hydrogenembrittlement can sometimes be prevented by the elimination of thesecontaminants from the system or by the addition of air. However, theseprocedures are seldom feasible and are usually quite expensive. Hydrogenembrittlement has also been known to occur in metal picklinginstallations, such as where steel springs are processed. In such casesthe springs which are produced are usually subject to premature failureand other defects.

It is an object of the present invention to prevent hydrogenembrittlement of metals.

Another object is to protect metallic structures and objects bypreventing electrochemical deposition of hydrogen on metallic surfacesof these structures and objects.

A further object is to overcome the normal electrochemical potentialswhich are generated when metallic surfaces are in contact with corrosivesolutions which tend to deposit hydrogen by rendering these surfacessufiiciently anodic to a cathode.

An additional object is to maintain metallic surfaces, particularlysteel, at an anodic electrical potential rela tive to a solution incontact with the surface, thereby preventing hydrogen embrittlement ofthe metal.

These and other objects of the present invention will become apparentfrom the description which follows. In the present invention, a metallicobject, such as a steel vessel or other container holding a solutionwhich tends to deposit hydrogen on the vessel surface by electrochemicalreaction is protected against subsequent hydrotion systems now in fairlycommon use.

gen embrittlement of the metal in a novel manner. An electricalpotential of suitable magnitude is maintained between the vessel walland the solution. This potential is established by immersing a suitablecathode in the solution, and providing an electrical circuitbetween thevessel wall and the cathode using direct electric current from anexternal source. Thus the vessel wall is maintained anodic in charge,which is the reverse of cathodic protec- The method of the presentinvention will usually cause an induced electrochemical corrosion,however, current density is regulated so that this corrosion is of a loworder of magnitude and can be tolerated, while the far more seriousproblem of hydrogen evolution and subsequent rapid failure due toembrittlement is eliminated. Since the vessel wall is anodic inelectrical potential relative to the solution, it becomes impossible forhydrogen to deposit on the Wall since any tendency for hydrogen ions tolose their charge and form hydrogen atoms is artificially reversed bythe strong anodic or positive potential of the wall. Thus negativelycharged particles are preferentially discharged against the wall ratherthan against hydrogen ions, and no hydrogen film can form on themetallic surface of the wall.

The magnitude of the anodic potential required for the prevention ofhydrogen deposition and consequent metal embrittlement is a function ofthe operating conditions encountered. Thus the magnitude of anodicpotential required must be empirically determined for each application.In any case, sufiicient electric potential is applied to the metallicstructure being protected from hydrogen embrittlement, to maintain alllocal areas of the surface at a positive potential. In general, therequired minimum voltage potential between the solution and any point onthe surface of the structure to be protected will be on the order of 0.8volt. That is, every point on the surface will be maintained positive byat least 0.8 volt relative to a copper-saturated copper sulfate halfcell in the electrolyte. Excessive voltage is undesirable, since it willnot produce any further protection and will also cause additionalinduced corrosion.

This system may be applied to vessels which are also coated, thusproviding protection against hydrogen ernbrittlement at breaks in thecoating. It should be noted that coated metallic surfaces requireconsiderably less amperage for protection than comparable uncoatedsurfaces.

The method of the present invention was utilized at a commercialchemical plant engaged in the production of acrylonitrile. Muchdifficulty had been encountered due to hydrogen embrittlement of steelvessels, since the acrylonitrile reacted with the metal vessel walls toproduce a hydrogen film which in turn formed gaseous compounds withnon-metallics in the grain boundaries of the metal. The typical hydrogenembrittlement which took place thus resulted in a deterioration andfailure of the vessel walls. The acrylonitrile contained slight amountsof potassium cyanide, acetic acid and phosphoric acid as principalimpurities, with a total impurity content of about 6%.

The vessel walls were made anodic with controlled amounts of electricalcurrent, which completely eliminated hydrogen deposition and preventedany subsequent hydrogen embrittlement. A minimum current density ofmilliarnperes per square foot was maintained on that portion of thesurface of the vessel in contact with the crude acrylonitrile. Thevoltage required to maintain this current density was a function of theresistivity of the solution which varied with impurity content and otheroperating factors. An operating voltage range between about 1 to 10volts was generally employed. This applied electrical potential alsocaused an acid to form which was corrosive. However, this inducedcorrosion amounted to only 0.0025 inch/year and was thus aninsignificant development which was readily tolerated since hydrogenembrittlement had been eliminated. The service life of the protectedprocess vessels was thus greatly extended.

A quantitative laboratory study of the effectiveness of the anodicprotection method was also made, based on the effectiveness of theaforementioned industrial application. A synthetic crude acrylonitrilesolution was prepared, containing 600 cc. acrylonitrile plus 14 gramseach of potassium cyanide and acetic acid, 1 cc. of 85% phosphoric acid,and 4 cc. water. Two test pieces of mild steel, each having 0.88 squaredecimeter of surface area, were immersed in this solution for a five daytest period. One steel test piece was unprotected, while the other wasconnected to a duriron cathode immersed in the solution so that anelectrical potential was generated and maintained by galvanic actionwhich ke t the second test piece anodic in potential relative to thesolution. After the five day test period the two test pieces wereremoved from the solution and weighed to determine whether theelectrically induced corrosion was of appreciable magnitude. Theprotected piece had a weight loss of 0.2136 gram, which corresponded toa corrosion rate of 0.009 inch/year. The unprotected piece had a weightloss of 0.2396 gram, which corresponded to a slightly higher corrosionrate of 0.010 inch/year. Thus it is evident that, within the limits ofexperimental accuracy, no increase in corrosion rate was caused by theanodic proection method. The two test pieces were then qualitativelytested for hydrogen absorption by immersion in hot oil and observationof hydrogen evolution. At the elevated temperature, any hydrogen presentis driven off as bubbles. No absorbed hydrogen was observed from theprotected test piece, while the unprotected piece showed considerableevolution of bubbles of hydrogen. Thus the anodic protection procedurehad effectively prevented hydrogen absorption in this test case, withnegligible induced corrosion.

The method of the present invention should be distinguished from anodicprotection technology of the prior art such as US. Patent No. 2,377,792in which anodic potential is employed to preserve protective oxide filmson materials such as stainless steel. In these cases a differentphenomenon and mechanism is involved, since film preservation by anodicprotection as practiced in the prior art is directed to prevention ofcorrosion by chemical attack and subsequent total metal wastage. Itshould be noted that the prior art did not comprehend the utilization ofanodic protection against hydrogen embrittlement, since in numerousinstances such as the aforementioned acrylonitrile application theconventional chemical type of corrosion is not serious. Thus in suchcases where conventional corrosion phenomena are not serious problems,but hydrogen embrittlement is a problem, application of anodicprotection has heretofore not been comprehended by the prior art.

We claim:

1. In the process of acrylonitrile synthesis, the method of preventinghydrogen deposition on ferrous metal surfaces in contact withacrylonitrile solution, which comprises immersing an inert electricalconductor in said acrylonitrile solution, and establishing an electricalpotential in the range of 1 to 10 volts between said conductor and saidferrous metal surface, whereby said surface is maintained continuouslyanodic in electrical potential relative to said solution, saidelectrical potential serving to establish a current density such thatthe ferrous metal surface is continuously dissolved into said solutionby induced corrosion, said induced corrosion being of a small andessentially negligible order of magnitude.

2. Method of claim 1, in which said electrical potential is regulated toa magnitude at which an average current density of about 5 milliarnperesper square foot is maintained on said surface.

References Cited in the file of this patent UNITED STATES PATENTS1,435,436 Slepian Mar. 4, 1924 1,513,824 Kasley Nov. 4, 1924 1,663,564Rich Mar. 27, 1928 1,731,269 Rich Oct. 15, 1929 1,825,477 Reichart Sept.29, 1931 2,057,274 Mayhew Oct. 13, 1936 2,360,244 McAuneny Oct. 10, 19442,377,792 Lawrence et al. June 5, 1945 2,576,680 Guitton Nov. 27, 19512,726,204 Park Dec. 6, 1955 2,886,497 Butler May 12, 1959 OTHERREFERENCES Edeleanu: Metallurgia, September 1954, pp. 113- 116.

Evans: Metallic Corrosion Passivity and Protection (1948), pages 84-85.

1. IN THE PROCESS OF ACRYLONITRILE SYNTHESIS, THE METHOD OF PREVENTINGHYDROGEN DEPOSITION OF FERROUS METAL SURFACES IN CONTACT WTIHACRYLONITRILE SOLUTION, WHICH COMPRISES IMMERSING AN INERT ELECTRICALCONDUCTOR IN SAID ACRYLONITRILE SOLUTION, AND ESTABLISHING AN ELECTRICALPOTENTIAL IN THE RANGE OF 1 TO 10 VOLTS BETWEEN SAID CONDUCTOR AND SAIDFERROUS METAL SURFACE, WHEREBY SAID SURFACE IS MAINTAINED CONTINUOUSLYANODIC IN ELECTRICAL POTENTIAL RELATIVE TO SAID SOLUTION, SAIDELECTRICAL POTENTIAL SERVINCE TO ESTABLISH A CURREN DENSITY SUCH THATTHE FERROUS METAL SURFACE IS CONTINUOUSLY DISSOLVED INTO SAID SOLUTIONBY INDUCED CORROSION, SAID INDUCE CORROSION BEING OF A SMALL ANDESSENTIALLY NEGLIGIBLE ORDER OF MAGNITUDE.